fix(Deferred): clear resumes before resuming waiters on completion - #7364

Fixes #7365

Summary

Completing a Deferred whose completion effect carries an interrupt cause can permanently starve later waiters. doneUnsafe iterates self.resumes live:

if (self.resumes) {
  for (let i = 0; i < self.resumes.length; i++) {
    self.resumes[i](effect)
  }
  self.resumes = undefined
}

When the completion effect is e.g. Exit.failCause(Cause.interrupt()), resuming a suspended waiter kills that waiter fiber synchronously inside resume(effect). The dying waiter's Deferred.await cleanup then runs while self.resumes is still defined and splices its own entry out of the array mid-iteration. The next waiter shifts down into the just-visited index, the loop index moves past it, and that waiter is never resumed — it hangs forever.

Concretely: with two suspended waiters, waiter 0 resumes (and dies of interruption), its cleanup splices resumes[0], waiter 1 becomes resumes[0], the loop advances to i = 1 against length = 1, and waiter 1 is skipped. Observed empirically in [email protected].

This is the starvation counterpart to #7194: that fix guarded the _await cleanup against running after completion (resumes === undefined), but the cleanup still mutates the array when it fires during the completion loop.

Fix

Snapshot and clear resumes before resuming, matching the swap-before-resume pattern Latch already uses for exactly this reentrancy hazard:

if (self.resumes) {
  const resumes = self.resumes
  self.resumes = undefined
  for (let i = 0; i < resumes.length; i++) {
    resumes[i](effect)
  }
}

Clearing first also makes a dying waiter's cleanup a no-op during completion (the resumes === undefined guard from #7193 short-circuits), so the loop iterates a stable snapshot.

All completion paths (done, complete, completeWith, fail, die, interrupt, into, and external doneUnsafe callers in PubSub, ScopedCache, FiberMap, FiberHandle, Socket) funnel through doneUnsafe, so this single change covers them. Queue resumes takers via Set iteration and Latch already snapshots, so neither shares the bug.

Reproducer

The added test forks two waiters, lets both suspend, then completes the Deferred with Exit.failCause(Cause.interrupt()). On unpatched main the second waiter never settles and the test's timeout fires; with the fix both waiters settle with the interrupt cause.

Testing

• pnpm vitest run --project effect test/Deferred.test.ts — new test fails before the fix, all 16 pass after
• pnpm vitest run --project effect test/Latch.test.ts test/PubSub.test.ts test/Queue.test.ts test/ScopedCache.test.ts test/FiberMap.test.ts test/FiberHandle.test.ts — 204 passed
• pnpm check in packages/effect, repo lint on touched files